QValueVector in Qt 3(.3.8) is broken

[fear]

I have put a lot of effort to rewrite huge part of my app to work with QValueVector but all in vain. Below I present proof of this broken component. Correct me if I'm wrong and I'll be happy.

My code:


qDebug(">");
playlistsList::iterator i, j;
i = playlists.begin();
j = i;
i += where;

//qDebug(playlists.at(which).name);
//qDebug((*i).name);

for (int i = 0 ; i != playlists.count() ; i++, j++)
{
qDebug(playlists.at(i).name + " " + (*j).name);
}

qDebug("i'll insert " + playlists.at(which).name + " before " + (*i).name);

playlists.insert(i, playlists.at(which));

for (int i = 0 ; i != playlists.count() ; i++)
qDebug(playlists.at(i).name);

qDebug("after redundant removed");

i = playlists.begin();
j = i;
i += (which + 1);

playlists.erase(i);

for (int i = 0 ; i != playlists.count() ; i++, j++)
qDebug(playlists.at(i).name + " " + (*j).name);

qDebug("");
qDebug("");


Results after first execution of above code are correct:


>
aa aa
bb bb
i'll insert bb before aa
bb
aa
bb
after redundant removed
bb bb
aa aa


Second execution gives INCORRECT result:


>
bb bb
aa aa
i'll insert aa before bb
bb
bb
aa
after redundant removed
bb bb
bb bb


Can anyone help me?

[jacek]

Could you post a minimal compilable example?

[fear]

/**
Broken QValueVector demo (Qt 3.3.8, GCC 4.1.1)
@author amdfanatyk

compilation: c++ qvalvecshit.cpp -o valvec -lqt-mt
**/

#include <qstring.h>
#include <qvaluevector.h>

struct elem
{
elem() {}
elem(const QString & _s1, const QString & _s2) { s1 = _s1; s2 = _s2; }

QString s1;
QString s2;
};

typedef QValueVector<elem> elemsList;

void printList(const elemsList & list)
{
qDebug("---");

for (int i = 0 ; i != list.count() ; i++)
qDebug(list[i].s1 + " " + list[i].s2);

qDebug("---");
}

void insertElem(elemsList & e)
{
if (e.count() < 2)
return;

elemsList::iterator i = e.begin();

printList(e);

QString e1s1 = e.at(1).s1;
QString e0s1 = (*i).s1;

qDebug("*- i'll insert " + e1s1 + " before " + e0s1);

e.insert(i, e.at(1));

printList(e);

if ((e.at(0).s1 != e1s1) || (e.at(1).s1 != e0s1))
qDebug("Ooops!");
else
qDebug("OK");
}

int main()
{
elemsList elems;

elem e1("a", "aa");
elem e2("b", "bb");

elems.append(e1);
elems.append(e2);

insertElem(elems);
insertElem(elems);

return 0;
}




---
a aa
b bb
---
*- i'll insert b before a
---
b bb
a aa
b bb
---
OK
---
b bb
a aa
b bb
---
*- i'll insert a before b
---
b bb
b bb
a aa
b bb
---
Ooops!

[jacek]

I've changed
e.insert(i, e.at(1));
to
elem el = e.at(1);
e.insert(i, el);
and it seems to work OK now.

IMO the problem is that at() returns a reference, which is valid only while the vector remains unchanged. But insert() has to make space for the new item, so it moves existing items and the reference becomes invalid.

[fear]

Thanks. Now it works but I don't think I'll use QValueVector and QTL. For me that's a bug. insert() should insert a copy of, so first it should make a copy and then insert it. To get it work I must make one copy and then vector makes another copy and I waste a lot of memory, it sucks. When I use my own engine I only change values of pointers - there is no need to make copies and remove redundant objects. With QPtrVector I think I'll have similar problems.

[jacek]

For me that's a bug. insert() should insert a copy of, so first it should make a copy and then insert it.
insert() itself is OK. The problem is that the item is passed by reference.


When I use my own engine I only change values of pointers - there is no need to make copies and remove redundant objects.
That's how QPtrVector works. QValueVector, just like std::vector, stores items, not pointers, so it has to copy them. But if you have your own collection that suits you better, just use it.